U.S. Pat. No. 3,516,460 Thrasher discloses a saw arbor having a plurality of circumferentially spaced, parallel, outwardly projecting semi-cylindrical splines. A circular saw blade having a saw eye cut to match the arbor's cross-sectional shape is slidably mounted on the arbor. As the arbor is drivingly rotated, the spline's leading edges tend to make point contact with the lower forward corners of the corresponding semi-circular cutouts in the saw eye. This significantly increases wear on the eye, and can ruin the saw blade well before the saw teeth themselves wear out. The arbor's splines also wear at an increased rate, as do the bearings which support the rotating arbor. Further, the saw blade tends to flutter at high speed, resulting in a wider kerf. All of these factors contribute to an increased need for saw blade changes, which is an expensive, labour-intensive operation with attendant loss of lumber production.
Involute-splined saw arbors were developed to overcome the foregoing problems. Involute splines have substantially flat forward, rearward and top faces. Gear cutting techniques are used to maintain the arbor's splines parallel to the arbor's longitudinal axis. As a result, instead of making mere point contact with the saw blade eye, an involute-splined arbor achieves land contact across substantially the entire forward face of each spline. This significantly reduces wear, saw flutter, etc. Further, because the arbor's splines are highly parallel to the arbor's longitudinal axis, the backlash tolerance between the arbor and the saw blade eye may be reduced in order to further reduce wear, flutter, etc.
Early prior art saw arbors, such as the aforementioned Thrasher arbor, were of solid, one-piece construction, with the saw blade being mounted over splines formed in the arbor itself. Modern saw arbors, of which U.S. Pat. No. 3,645,304 Thrasher is typical, have a mandrel on which a cylindrically-apertured sleeve is slidably mounted, with the saw blade being mounted over splines formed around the sleeve's outer circumference. One or two longitudinally extending keyways are machined into the sleeve's aperture. The mandrel has a similar keyway. The keyways are aligned and a steel key is placed in the aligned keyways to position the sleeve relative to the mandrel.
Such arbors are subject to a number of problems. For example, the keyway machining process removes material from the mandrel and from the sleeve. The weight of the removed material is not precisely offset by the steel key placed in the keyway. This results in rotational imbalance, which can degrade sawing accuracy when the arbor and saw blade are driven at high rotational speeds.
A further "ovality problem" arises upon heat treatment of an arbor having a keyed mandrel and sleeve. In particular, such arbor sleeves naturally and unavoidably tend to assume an oval (i.e. out of round) cross-sectional shape following heat treating. If the arbor sleeve is out of round, then the saw blade eye cannot be formed to achieve a minimum tolerance, orientation-independent, fit on the arbor.
More particularly, the sleeve portion of a typical prior art arbor having a keyed mandrel and sleeve is commonly made by forming a series of external splines on the outer surface of a piece of cylindrical stock. The outer surface of the splined piece is then heat treated so that the splines will be able to resist wearing caused by saw eyes moving relative to the splines. A cylindrical aperture is then rough bored axially through the heat treated, splined piece and one or two longitudinally extending keyways are cut in the bored aperture, to mate with corresponding keyways formed in a mandrel. The aperture is then finish bored to correct ovality in the aperture's internal circumference which is observed after the keyways are cut.
The heat treating step creates stresses in the material of the splined piece. When keyways are cut in the splined piece these stresses give rise to the aforementioned ovality problem, causing the piece to deform so that it becomes out of round. The exact amount and/or direction of deformation is not possible to predict, and varies from sleeve to sleeve. The ovality problem affects both the internal and external circumferences of the sleeve portions of prior art arbors having a keyed mandrel and sleeve. One can correct the ovality of the sleeve's internal circumference by finish boring the sleeve aperture, as above. But, the ovality of the external circumference of the sleeve portion of a prior art arbor having a keyed mandrel and sleeve can not be eliminated without resorting to expensive grinding techniques. Prior art arbors having a keyed mandrel and sleeve, including sleeves having keyways formed in accordance with the description of "ANSI standard keys and keyseats" set forth on page 2234 of the 23rd edition of Machinery's Handbook, invariably exhibit the ovality problem.
To accommodate the ovality problem in a prior art arbor having a keyed mandrel and sleeve it is necessary to provide saw blades having eyes which fit the sleeve to relatively loose backlash tolerances. Such tolerances are typically no better than about 0.007" to 0.015". This loose tolerance allows sawmill workers to fit any saw blade on any arbor sleeve without regard to orientation of the saw eye relative to the sleeve. One could fit a saw eye more closely to an out of round sleeve by making the saw eye out of round to match the sleeve. However, the saw eye would then only fit the sleeve in one or two orientations. In some orientations, the saw eye would not fit over the sleeve because the sleeve's outer diameter at certain points would be greater than the inner diameter of the saw eye.
Saw blade eyes are conventionally formed using laser cutting techniques. After the saw eye is laser cut, the saw blade is heat treated to the desired hardness. But, the heat treating process unavoidably distorts the shape of the saw blade eye. This is another reason why saw eyes are conventionally laser cut to backlash tolerances of no better than about 0.007" to 0.015". If the saw eye were cut to a closer tolerance, then distortion introduced during the heat treating process might prevent the saw blade from fitting in any orientation on any arbor.
To illustrate the foregoing, FIG. 1A shows a prior art saw blade A mounted on a prior art arbor sleeve B before cutting of any keyway in sleeve B. If no keyways are cut in sleeve B the aforementioned ovality problem does not arise. Accordingly, as seen in FIG. 1A, saw eye splines C fit sleeve splines D with the same tolerance, regardless of the radial orientation of the saw eye relative to sleeve B. FIG. 1B shows a prior art saw blade E mounted on a prior art arbor sleeve F in which keyway G has been cut. Due to the aforementioned ovality problem, the external diameter of sleeve F measured along horizontal axis H as viewed in FIG. 1B exceeds the external diameter of sleeve F measured along vertical axis I as viewed in FIG. 1B. This effect is exaggerated in FIG. 1B, to better show that saw eye splines J fit sleeve splines K more closely in the region near horizontal axis H. Saw eye splines J do not fit sleeve splines K as closely in the region near vertical axis I.
Modern sawmills typically employ many circular saws, each of which undergo frequent saw blade changes. It is impractical to maintain a separate inventory of blades having eyes shaped to fit specific arbors in specific orientations; and/or take the time to orient a saw blade's eye to achieve minimum tolerance fit on an arbor. This is why circular saw blade eyes are formed to a loose tolerance, which may be no better than about 0.007" to 0.015", as previously explained. This loose tolerance allows the sawmill workers to fit any blade on any arbor without regard to orientation of the saw eye relative to the arbor. However, a necessary consequence is increased wear and sawing inaccuracy, as discussed above.
The present invention eliminates the aforementioned ovality problem by providing a plurality of splines formed integrally with the arbor sleeve and spaced circumferentially around the inner circumference of the sleeve's aperture. There are no keyways in the splined portions of the sleeve, so neither the internal nor the external circumferences of the splined sleeve become significantly deformed due to heat treating stresses. Therefore, it is possible to maintain much tighter tolerances between a saw blade eye and the externally splined portion of such a sleeve than is possible with a prior art arbor having a keyed mandrel and sleeve.